The proposed project employs a density comparator to measure the density difference between the arterial and venous blood of the pulmonary circulation as a means to evaluate the pulmonary filtration. Their density is measured by the density meter developed by Kenner's group. It consists of a U-shaped glass tube containing the blood and a feedback system to excite the tube to oscillate at its resonant frequency. This frequency is converted to a signal proportional to the density of blood flowing through the tube. The proposed density comparator uses two density meters for which their resonant frequencies are matched by the use of a micrometer set-up. By mixing the two resonant signals electronically, we measure the subharmonic frequency for the computation of the density difference between the blood flowing through the two density meters. Because of a different density for the fluid filtrate and the red blood cells, the fluid loss by filtration causes a constant change in the density between the arterial and venous blood. For the proposed system, the effect of biological variations is minimized and the accuracy to detect the difference between two resonant frequencies is enhanced. A simple minded analysis shows that the comparator could have sufficient sensitivity to determine the rate of pulmonary filtration. This technique is microinvasive and can determine rapidly and directly the loss of fluid from the vascular to the tissue compartment. If the desirable accuracy is confirmed by the proposed study, the density comparator will be far superior than all existent techniques for monitoring pulmonary filtration and identifying edema.